Present commercial engine configurations combine aerodynamic turning vanes and structural struts, used to support outer cases such as the fan containment case, into a single row of airfoils. This configuration utilizes a large number of airfoils relative to the fan blade count for acoustic reasons. With a high count of structural airfoils supporting the outer cases, a uniform stiffness is created at this engine axial station.
When a case is supported by a uniformly stiff structure, the case has the potential to exhibit vibratory modes with low to high nodal diameters. One or more of these vibratory modes may promote the potential for coincidence if they have the same frequency as an equivalent nodal diameter pattern on the rotor and the rotor is engaging the tip abradable system. Such an event is referred to as coincidence or a coincidence event. A coincidence event may occur if the energy transmission through rubbing continues to amplify the rotor to case interaction.
Conventional practice is to tune case modes out of the operating range by modifying the case thickness distribution or profile. This may be impractical for some modes, considering the weight addition required. In this situation, the standard approach to prevent coincidence is to open fan tip clearance. Opening fan tip clearance avoids the three hundred and sixty degree) (360° rubs, which can generate coincidence events. To avoid the potential for coincidence, fan tip clearances are opened up to prevent fan rubs greater than ninety degrees) (90°, that is, assuming difficult tolerance stack-up conditions and all potential field events. Typically, this tip clearance increase lowers fan blade efficiency by up to one percent (1%), increasing thrust specific fuel consumption by 0.5 to 0.6 percent, which significantly increases engine fuel burn.
Consequently, there exists a need for a fan rotor that addresses coincidence avoidance without sacrificing fan blade efficiency.